1. Field of the Invention
The present invention relates to electronic ceramic compounds in the BaO-TiO.sub.2 system. Different phases in the BaO-TiO.sub.2 system are of strategic and industrial importance for transducer and microwave dielectric applications. The barium titanate (BaTiO.sub.3) phase exhibits ferroelectric behavior, i.e., a spontaneous alignment of electric dipoles within the material which is analogous to the alignment of magnetic dipoles in ferromagnetic materials. The BaTiO.sub.3 phase has been used as a transducer material and as a basis for very high dielectric constant ceramics in ultrasonic devices, microphones, phonograph pickups, accelerometers, strain gauges and sonar devices. The BaTi.sub.4 O.sub.9 phase is used as a microwave dielectric since it has a relatively high dielectric constant and high Q (low dielectric loss) combined with a very low temperature dependence of the dielectric constant (TCK). Recent investigations have shown that the Ba.sub.2 Ti.sub.9 O.sub.20 phase exhibits even more desirable properties for microwave dielectrics, but reproducible ceramic processing parameters have so far been very difficult to formulate. Also, these investigations have indicated that phases with higher BaO:TiO.sub.2 ratios than 2:9 would exhibit very useful high frequency dielectric properties.
2. Description of the Prior Art
In U.S. Pat. No. 4,058,592, a process for the preparation of sub-micron barium-titanate powders is disclosed. The process involves mixing a carbohydrate material with a solution of barium and titanium compounds, at least one of which contains a chloride ion, followed by ignition and calcination of the mixture. In this process, the presence of chloride ions during ignition precludes the direct formation of barium titanate and thus the chloride ions must be removed prior to ignition.
In U.S. Pat. No. 4,061,583, a process for the preparation of barium titanate is disclosed. The process involves adding an aqueous solution containing titanium and barium ions to an aqueous alkaline solution containing hydrogen peroxide, recovering a precipitated complex peroxide corresponding to barium titanate from the resulting mixture, and decomposing the precipitate by heating. Amorphous substantially stoichiometric barium titanate is thereby obtained.
In U.S. Pat. No. 4,086,649, several processes for the preparation of stoichiometric barium titanate are disclosed. A first process involves mixing an alcoholic solution of tetraisopropyl titanate and corn syrup with an aqueous solution of barium acetate, followed by drying and igniting the resultant mixture to form barium titanate powders. A second process involves mixing an aqueous solution of barium acetate and corn syrup with an alcoholic solution of triethanolamine titanate, followed by drying and igniting the resultant mixture to form barium titanate powders. A third process involves mixing an aqueous solution of barium acetate and sucrose with an alcoholic solution of triethanolamine titanate, followed by drying and igniting the resultant mixture to form barium titanate powders.
A first conventional solid-state technique for the preparation of barium-titanate powders involves mixing TiO.sub.2 powder with BaCO.sub.3 powder and calcining the resultant mixture at 800.degree.-1100.degree. C. The calcined mixture is then pulverized into a fine powder by conventional mortar and pestle, ball-milling or jet-milling techniques. This fine powder is pressed into a desired shape and fired at 1300.degree.-1450.degree. C. This process yields a powder of relatively low-purity that is used to process dielectric ceramics for applications requiring less demanding performance.
A second conventional solid-state technique for the preparation of barium-titanate powders involves precipitating a BaTi-oxalate precursor from oxalic acid and metal chlorides. This precursor is then calcined at elevated temperature to crystallize BaTiO.sub.3. These powders are of higher purity than the powders of the first conventional technique but chloride contamination is difficult to eliminate completely.